As easy-to-access and easy-to-produce hydrocarbon resources are depleted, there is an increased demand for more advanced recovery procedures. One such procedure is steam assisted gravity drainage (SAGD), a procedure that utilizes steam in conjunction with two spaced apart wellbores. Specifically, SAGD addresses the mobility problem of heavy oil in a formation through the injection of high pressure, high temperature steam into the formation. This high pressure, high temperature steam reduces the viscosity of the heavy oil in order to enhance extraction. The injection of steam into the formation occurs from a first wellbore (injector) that is drilled above and parallel to a second wellbore (producer). As the viscosity of the heavy oil in the formation around the first wellbore is reduced, the heavy oil drains into the lower second wellbore, from which the oil is extracted. Other advanced recovery procedures include thermal Assisted Gravity Drainage (TAGD), Toe to Heal Air Injection (THAI), Vaporized Hydrocarbon Solvent (VAPEX) production and Fire Flooding. In all of these advanced recovery procedures, the precise placement of adjacent local cased wellbores is an important aspect of the process.
More specifically, the two wellbores are typically drilled at a fixed distance of only a few meters from one other. The placement of the injector wellbore needs to be achieved with very small margin in distance. If the injector wellbore is positioned too close to the producer wellbore, the producing wellbore would be exposed to direct communication of steam at very high pressure and temperature. If the injector wellbore is positioned too far from the producer wellbore, the efficiency of the particular advanced recovery process may be reduced. In order to assist in ensuring that the second wellbore is drilled and positioned as desired relative to the first wellbore, a survey of the two wellbores in the formation is often conducted. These surveying techniques are traditionally referred to as “ranging”. Based on ranging, the trajectory of the second wellbore, i.e., the wellbore being drilled, can be altered to ensure the fixed distance between the wellbores is maintained.
One common ranging technique employs electromagnetic (EM) systems and methods to determine direction and distance between two wellbores and ensure the fixed distance between the wellbores is maintained. In EM ranging systems, an elongated, conductive pipe string, such as the wellbore casing, is disposed in one of the wellbores. This wellbore is typically referred to as the “target” wellbore and usually represents the producer wellbore. In any event, a current is applied to this conductive pipe string in the target wellbore by a low-frequency current source. The current flow along the conductive pipe string and result in EM fields around the target wellbore. These EM fields around the target wellbore are measured using an electromagnetic field sensor system disposed in the other wellbore, which is typically the wellbore in the process of being drilled. This second wellbore usually represents the producer wellbore. The measured magnetic field can then be utilized to determine distance, direction and angle between two wellbores, and if necessary, alter the trajectory of the wellbore being drilled. Ranging systems in which a current is injected into the target wellbore in order to induce a magnetic field around the target wellbore are referred to as “active” ranging systems.
The existing electromagnetic ranging methods have been used successfully to place an injector wellbore at fixed distance from a producer wellbore in operations such as SAGD. In such operations, it has been observed that if the wellbores are closer than optimal, direct fluid communication in the form of steam may be observed resulting in production of steam as opposed to hydrocarbons. Likewise, if the wellbores are farther than optimal, mobility of hydrocarbons will not be increased high enough for production. In any case, wellbores are typically placed using EM ranging at fixed separation distance from one another, whereby the separation distance is chosen to optimize and balance these two above-mentioned considerations. However, due to variations in the geology and formation characteristics, a constant distance is usually not always optimal for production. Furthermore, the distance may not be selected properly, or may be selected too conservatively which can lead to non-optimal production.